1. Field of the Invention
This invention relates to a novel process for preparing hexamethylcyclotrisilazane.
2. Description of the Prior Art
As is known in the art, hexamethylcyclo-trisilazane is effective as a liquid phase silification agent for positive resists comprised mainly of novolak resins and is useful as imparting a plasma resistance to the resin (Photochemistry of Silicon, by Hiroyuki Hiraoka, 1990 (No. 1) p.3-19, and Stephan E. Greco, S. Miura, Journal of Electrochem Society, 1991 (No. 3), pp. 810-814).
Known process of preparing hexamethyl-cyclotrisilazane of the following formula (1) include a so-called direct ammonolysis process wherein dimethyldichlorosilane and ammonia gas are reacted with each other (S. D. Brewer and C. P. Haver, J. Am. Chem. Soc., 70 3880 (1948). ##STR2##
However, this process has the serious problem that because there is secondarily produced, aside from hexamethylcyclotrisilazane, octamethylcyclotetrasilazane of the following formula (2) in large amounts (about 10 to 30%), the octamethylcyclotetrasilazane side product has to be properly treated on mass-production of hexamethylcyclotrisilazane ##STR3##
More particularly, the octamethylcyclo-tetrasilazane has no specific utility, to which little or no attention has been paid for a long while. In recent years, there has been proposed an interesting process that the useful cyclic trimer is prepared from the cyclic tetramer silazane side product through cracking in the presence of a specific type of catalyst in an atmosphere of hydrogen (Japanese Patent Publication No. 63-58838).
However, this process inconveniently makes use of not only hydrogen which has a wide explosion limit and is thus difficult to handle, but also an expensive catalyst such as nickel, platinum or ruthenium. Hence, the processes is not useful for industrially preparing hexa-methylcyclotrisilazane which is a cyclic trimer.